What does the story of our species tell us? By this I mean the narrative made visible by science, not the archaic version soaked in religion and ideology. I believe the evidence is clear enough to tell us this much: We were created not by a supernatural intelligence but by chance and necessity as one species out of millions in Earth's biosphere. Hope and wish for otherwise as we will, there is no evidence of an external grace shining down upon us, no demonstrable destiny or purpose assigned us, no second life vouchsafed us for the end of the present one. We are, it seems, completely alone.

And that in my opinion is a very good thing. It means we are completely free. And we can more easily trace the irrational beliefs that so unjustifiably divide us. Humanity arose as an accident of evolution, a product of random mutation and natural selection. Our species was just one end point out of many twists and turns in a single lineage of Old World primates (prosimians, monkeys, apes, humans) of which there are today several hundred other native species, each a product of its own twists and turns. We might easily have remained just another australopith with an ape-sized brain, collecting fruit and grabbing at fish, eventually to suffer extinction.

For the 400 million years that large animals have occupied the land, Homo sapiens has been the only one to evolve intelligence high enough to create a civilization. Our genetically nearest relatives, the chimpanzees, came closest. The human and chimpanzee lineages split from a common stock in Africa about 6 million years ago.

Roughly 200,000 generations have passed, plenty of time for natural selection to force a series of major genetic changes. The prehumans possessed certain advantages that biased the direction of their subsequent evolution. These included, at the beginning, a partly arboreal life and free use of the forelimbs that went with it. This archaic condition was then altered to a primarily ground-dwelling life.

Also in place as biasing conditions were large-brained ancestors and an immense continent with a mostly equitable climate and extensive grassland interspersed with open dry forest. In later years the favoring conditions included frequent ground fires that promoted fresh growth in herbaceous and shrubby plants. Also, and more important, the fires made possible an eventual dietary shift to cooked meat. This rare combination of circumstances during the evolutionary run-up, combined with luck (no devastating climate change, volcanic eruptions or severe pandemics), rolled the dice in favor of the early humans.

Godlike, their descendants have saturated a large part of Earth, and altered to varying degree the remainder. We have become the mind of the planet and perhaps our entire corner of the galaxy as well. We can do with Earth what we please. We chatter constantly about destroying it -- by nuclear war, climate change, an apocalyptic Second Coming foretold by Holy Scripture.

Human beings are not wicked by nature. We have enough intelligence, goodwill, generosity and enterprise to turn Earth into a paradise both for ourselves and for the biosphere that gave us birth. We can plausibly accomplish that goal, at least be well on the way, by the end of the present century. The problem holding everything up thus far is that Homo sapiens is an innately dysfunctional species. We are hampered by the Paleolithic Curse: genetic adaptations that worked very well for millions of years of hunter-gatherer existence but are increasingly a hindrance in a globally urban and techno-scientific society. We seem unable to stabilize either economic policies or the means of governance higher than the level of a village.

Further, the great majority of people worldwide remain in the thrall of tribal organized religions, led by men who claim supernatural power in order to compete for the obedience and resources of the faithful. We are addicted to tribal conflict, which is harmless and entertaining if sublimated into team sports, but deadly when expressed as real-world ethnic, religious and ideological struggles.

There are other hereditary biases: Too paralyzed with self-absorption to protect the rest of life, we continue to tear down the natural environment, our species' irreplaceable and most precious heritage. And it is still taboo to bring up population policies aiming for an optimum people density.

Our species' dysfunction has produced the hereditary myopia of which we are all uncomfortably familiar. People find it hard to care about other people beyond their own tribe or country, and even then past one or two generations. It is harder still to be concerned about animal species -- except for dogs, horses and others of the very few we have domesticated.

Our religious, political and business leaders mostly accept supernatural explanations for human existence. Even if privately skeptical, they have little interest in stirring up the populace, from whom they draw power and privilege. Scientists who might contribute to a more realistic worldview are especially disappointing. Largely yeomen, they are intellectual dwarves content to stay within the narrow specialties for which they were trained and are paid.

Some of the dysfunction of course comes from the youthful state of global civilization, which is still a work in progress. But the greater part is due simply to the fact that our brains are poorly wired. Hereditary human nature is the genetic legacy of our prehuman and Paleolithic past -- the "indelible stamp of our lowly origin" as identified by Charles Darwin, first in anatomy ("The Descent of Man," 1871) and then in the facial signals of emotion ("The Expression of the Emotions in Man and Animals," 1872). Evolutionary psychologists have pressed on to explain the role of biological evolution in gender differences, child mental development, status ranking, tribal aggression and even dietary choice.

As I've suggested in previous writing, the chain of causation runs yet deeper, extending all the way to the level of the biological organization on which natural selection works. Selfish activity within the group provides competitive advantage but is commonly destructive to the group as a whole. Working in the opposite direction from individual-level selection is group selection -- group versus group. When an individual is cooperative and altruistic, this reduces his advantage in competition with other members but increases the survival and reproduction rate of the group as a whole. In a nutshell, individual selection favors what we call sin and group selection favors virtue. The result is the internal conflict of conscience that afflicts all but psychopaths, estimated fortunately to make up only 1 to 4 percent of the population.

The products of the opposing two vectors in natural selection are hardwired in our emotions and reasoning, and cannot be erased. Internal conflict is not a personal irregularity but a timeless human quality. No such conflict exists or can exist in an eagle, fox or spider, for example, whose traits were born solely of individual selection, or a worker ant, whose social traits were shaped entirely by group selection.

The internal conflict in conscience caused by competing levels of natural selection is more than just an arcane subject for theoretical biologists to ponder. It is not the presence of good and evil tearing at one another in our breasts. It is a biological trait fundamental to the human condition, and necessary for survival of the species. The opposed selection pressures during human evolution produced an unstable mix of innate emotional responses. They created a mind that is continuously and kaleidoscopically shifting in mood -- variously proud, aggressive, competitive, angry, vengeful, venal, treacherous, curious, adventurous, tribal, brave, humble, patriotic, empathetic and loving. All normal humans are both ignoble and noble, often in close alternation, sometimes simultaneously.

The instability of the emotions is a quality we should wish to keep. It is the essence of the human character, and the source of our creativity. We need to understand ourselves in both evolutionary and psychological terms in order to plan a more rational, catastrophe-proof future. We must learn to behave, but let us never even think of domesticating human nature.

(Edward O. Wilson is a biologist and naturalist and the author of more than 20 books. This is an excerpt from "The Meaning of Human Existence," which will be published Sept. 15 by Liveright.)